National Weather Service United States Department of Commerce


Several changes and additions have been incorporated into the 1981-2010 Normals. Monthly temperature and precipitation normals are based on underlying data values that have undergone additional quality control. Monthly temperatures have also been standardized to account for the effects of station moves, changes in instrumentation, etc.

Unlike the 1971-2000 Normals, daily data were used extensively in the computation of daily temperature and precipitation normals as well as heating and cooling degree day normals, providing greater precision of intra-seasonal features. In previous installments, daily precipitation normals were computed as a spline fit through the monthly values. For 1981-2010, this metric will be replaced with a suite of metrics, including daily probabilities of precipitation as well as month-to-date and year-to-date precipitation normals. New products in the 1981-2010 Normals include normals derived from hourly data values.  Therefore, summing up the daily rainfall or degree day values (or adding up the daily temperature values) will not necessarily add up (or average out to, in terms of temperature data) to the monthly average for that value.